1 — Back toward the imam’s back: Valid.
2 — Face toward the imam’s face: Valid but disliked.
3 — Back toward the imam’s face: Invalid following because of being ahead of the imam in the relevant direction.
The outcome depends on direction and precedence, not on the words face and back in isolation.
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Prayer in, Around, and on the Roof of the Kaaba
1 — Back toward the imam’s back: Valid.
2 — Face toward the imam’s face: Valid but disliked.
3 — Back toward the imam’s face: Invalid following because of being ahead of the imam in the relevant direction.
The outcome depends on direction and precedence, not on the words face and back in isolation.